1. Field of the Invention
The present invention generally relates to the field of electrical and/or electronic devices. In particular, the present invention relates to solid state relays.
2. Description of Related Prior Art
Electrical and/or electronic devices of all kind are commonly very sensitive to interferences by ambient electromagnetic noise.
A solid state relay has all its components made from solid state devices and involves no mechanical movement. They are compatible with digital circuitry and have a wide variety of uses with such circuits.
Compared to electromechanical relays, solid state relays have several advantages including an increased lifetime, particularly at a high rate of switching; an elimination of contact bounce; decreased electrical noise; compatibility with digital circuitry; the ability to be used in explosive environments since there are no contacts across which arcs can form; and low voltage turn-on that reduces both the electromagnetic interference and stress on the attached load. The lack of physical contacts and moving elements also provides increased resistance to corrosion and the elimination of mechanical noise.
However, typical prior art solid state relays exhibit various disadvantages including the generation of a substantial amount of heat at a current above several amperes necessitating some form of cooling, and greatly increased production costs for multiple devices compared to single pole devices. Apart from the fact that typical prior art solid state relays usually are very costly to manufacture, they have high leakage current and are prone to false triggering.
The bottom line is that conventional solid state relays do not always produce reliable and satisfactory results. Furthermore, prior art solid state relays are relatively expensive and difficult to produce and operate because of their large number of internal components. Finally, prior art solid state relays are quite large compared to other electrical components, making their inclusion in widely-developed, miniature electrical devices a costly endeavor.
In this context, it should be noted that conventional solid state relays often comprise one or more switching devices which when used in direct current (DC) circuits require a single trigger pulse to turn the switching device ON. The switching device remains switched ON until the current drawn by the device falls below a threshold value, at which point the switching device turns OFF and requires a fresh trigger pulse before it will turn ON again.
When the switching device is used in an alternating current (AC) circuit, a continuous trigger signal is supplied to the switching device to prevent the device switching OFF when the current passes through a zero level. This is somewhat wasteful of power since the trigger circuit is drawing a continuous supply of current.
When the load which the device controls is an inductive load, like an electric motor, the continual switching produced by the device produces radio frequency interference. In many situations such interference is unacceptable and must be suppressed with a suppression circuit of the type which includes a suppression inductor and a suppression capacitor. These additional components may well double the cost of the device.
It is clear that a need exists for an improved solid state relay that overcomes the shortcomings of these prior art relays.